Originally Posted by Reynolds 531
"It is now 2025, and your Biopace you threw away, has been pulled from the dustbin and has been purchased for $250, by a guy riding an electric skateboard. He has a computer in his pocket that is more powerful by far than the computers that put a man on the moon, and he is assembling a bicycle to ride while his electric car is being charged ata public charging station."


If that were one of the "HP", "II", or "SG" variants of BioPace, that's what a crank of that quality might cost new in today's market, right?

I mention the three later variants of BioPace rings offered, only because every thread that ever popped up about BioPace gets the story wrong as far as which rings in which positions have which amount of non-roundness!

Summarizing;

The original BioPace large chainrings were all a much-rounder (only 3.5%) variation of radius, likely for chain control at the front derailer, while the other (smaller) ring(s) had a "wilder" 8% variation along the radius of the chain's path (so 8% variation in gear ratio as the crankarms moved through 180-degrees).

All of the newer (HP, II and SG) variants however used the much-milder 3.5% variation shape for all chainring positions.
The later (3.5%) rings are excellent even for racing (and happen to be my first choice for all racing disciplines).
With much less shape/radius variation, no shifting or chain-control limitations arise, and the cranks can be fully spun as far as I can tell. The Bio effect is at least slightly useful when having to accelerate from too tall of a gear as when perhaps recovering from having drifted off course (common in CX). The 3.5% variation is barely noticed, even initially!
Even the later (usually stamped) BioPace-SG rings (found on so many lower-cost bikes) are really good chainrings imo. The steel ones shift the best out of all of them and are particularly durable.

There is a limit as to how small of a middle or small BioPace chainring will fit on your particular bolt circle though, and the original BioPace 130mm BCD small ring could only be offered as small as 42T. The later (and rounder) 130mm BioPace small ring was usually offered as 40T (around the time that other makers began using 39T).
The smallest size middle 110mm BioPace ring that was ever offered was 36T across all variants, and the smallest 74mm BioPace ring offered was 26T across all variants.

The first of the rounder middle/small BioPace variants hit the market in I think about 1986, but I haven't bothered to check up on that.
This after many riders complained about the wilder 8% variation of the original BioPace small and middle chainrings (since the change going from the rounder big ring to the smaller ring(s) was immediately disturbing to one's spin, making the gearing change seem more large/abrupt than it was).
With that said, I can't imagine trying to race using the original BioPace setup (unless it was a criterium course where I never had to downshift off of the big ring).